1. Field of the Invention
This invention relates to a voltage regulator circuit which, by interrupting the excitation current supplied to the field winding of a generator, regulates the output voltage of the generator to a predetermined level; more particularly, this invention relates to such a voltage regulator circuit for an AC generator, especially suited to be installed in an automobile, which comprises a positive feedback circuit, consisting of a serial RC circuit, for stabilizing the repeated interrupting operations of the power transistor of the voltage regulator circuit coupled in series with the field winding of the generator.
2. Description of the Prior Art
Automobiles are generally provided with an AC generator the output of which is rectified by a rectifier circuit and is regulated to a predetermined voltage level by a voltage regulator circuit. In such case, the voltage regulator circuit, which interrupts the excitation current supplied to the field winding of the generator when the output voltage exceeds the predetermined level. Generally comprises a positive feedback circuit coupled across the collector of the power transistor, i.e., the transistor coupled in series with the field winding of the generator for interrupting the excitation current supplied thereto, and the base of the control transistor in the voltage detector circuit controlling the power transistor.
FIG. 1 shows a conventional voltage regulator circuit for an AC generator of an automobile. The AC generator or alternator 1 comprises an armature winding 101 and a field winding 102. A full-wave rectifier circuit 2 coupled to the output terminals of the generator 1 is provided with a main and an auxiliary positive output terminal 201 and 202, and with a grounded negative output terminal 203. The voltage regulator circuit 3 has a well-known circuit structure: A voltage divider consisting of serially connected resistors 301 and 302 is coupled across the auxiliary and the negative terminal 202 and 203 of the rectifier circuit 2, the junction between resistors 301 and 302 being coupled to the base of a control transistor 304 through a Zener diode 303. The base of the power transistor 305 coupled in series with the field winding 102 is coupled, on the one hand, to the junction between the key switch 5 and an indicator lamp 6 through a resistor 306; the base of power transistor 305 is coupled, on the other hand, to the collector of control transistor 304 having a grounded emitter. Further, a resistor 311 for supplying initial excitation current from the battery 4 to the field winding 102 is coupled in parallel with the indicator lamp 6. A diode 307 for absorbing and suppressing the surge generated by the inductance of the field winding 102 upon interruption of the excitation current is coupled across the field winding 102. Further, a serial circuit of a positive feedback resistor 308 and a positive feedback capacitor 309 is coupled across the collector of the power transistor 305 and the base of the control transistor 304, a biasing resistor 310 being coupled across the base and emitter of control transistor 304 for producing a forward and a reverse bias thereacross.
The operation of the circuit of FIG. 1 is well known and roughly as follows: When the key switch 5 is closed to start the engine of the automobile, initial excitation current is supplied from the battery 4 to the field winding 102 through the parallel circuit of inidcator lamp 6 and resistor 311, thereby placing the generator 1 in condition for generating power. At the same time, the inidcator lamp 6 is energized to indicate that no voltage is delivered from the generator 1 yet. When the engine is started thereafter to rotate the field winding 102, AC voltages induced in the armature winding 101 are rectified by the rectifier 2 into a DC voltage. When the voltage at the positive output terminals 201 and 202 rises to the level of the battery voltage, the lamp 6 is extinguished to indicate that the generation of power has begun properly. Thereafter, the regulation of output voltage is effected as follows: When the output voltage at the auxiliary terminal 202 exceeds a predetermined level to raise the voltage at the junction between resistors 301 and 302 above the Zener voltage of the Zener diode 303, the Zener diode 303 becomes conductive in the reverse direction to turn on control transistor 304, thereby turning off power transistor 305. Conversely, when the output voltage at the auxiliary terminal 202 falls under the predetermined level, the Zener diode 303 again becomes non-conductive to turn off control transistor 304, thereby turning on power transistor 305. The output voltage of the AC generator is thus controlled to the predetermined level.
In the above described operation, the positive feedback circuit consisting of a resistor 308 and a capacitor 309 contributes to the stable interrupting operation of the power transistor 305 as follows:
When the power transistor 305 is turned off, the voltage at the collector thereof rises abruptly from a substantial ground level. Thus, the capacitor 309 begins to be charged through resistor 3008, and the current charging the capacitor 309 supplies the base current of the control transistor 304, thereby helping the control transistor 304 to stay in the conductive state until the current charging the capacitor 309 is diminished. On the other hand, when the output voltage of the rectifier circuit 2 falls under the predetermined level to turn off the Zener diode 303 and the control transistor 304, thereby turning on the power transistor 305, the collector voltage of the power transistor 305 falls abruptly almost to the ground level. Thus, the previously charged capacitor 309 begins to discharge through the serial circuit formed by the positive feedback resistor 308, power transistor 305, and the biasing resistor 310, thereby giving a reverse bias across the emitter and the base of control transistor 304. This reverse bias, which subsists until the capacitor 309 is substantially discharged, helps the control transistor 304 to stay in the non-conductive state. In the above described manner, in both operations of turning on and turning off of the power transistor 305, the positive feedback circuit consisting of the feedback resistor 308 and the feedback capacitor 309 produces a forward and a reverse bias across the base and emitter of the control transistor 304, and helps the control transistor 304 to stay turned on or turned off for a period of time, thereby realising the stable interrupting operation of the power transistor 305.
The voltage regulator circuit of FIG. 1, however, has the following disadvantage: Since the internal combustion engine of the automobile comprises an ignition system generating extremely high voltages, voltage surges, which may reach almost 300 volts, may arrive at the collector of the power transistor 305 in the non-conductive state. These voltage surges occurring when the power transistor 305 is turned off are applied across the capacitor 309 through the resistors 308 and 310. Thus, the positive feedback capacitor 309 is required to withstand a voltage in the range of of from 150 to 300 volts. Further, in the designing of the voltage regulator circuit, a due consideration must be payed for the voltage surges, which makes the designing of the circuit difficult.